Aqueous mineral compositions are commonly employed in the coating of paper webs. Such compositions exhibit relatively high solids contents and viscosities but, nevertheless, usually include a considerable proportion of water. When such a composition is applied to a cellulosic paper web, the fibers take up water, weakening the web and rendering the composition itself, in the areas from which the water has preferentially migrated, somewhat higher in solids, and somewhat more viscous. Thus, both the coating and the web tend to change characteristics upon application of the composition to the web. Further, they tend to change in a manner which makes the attainment of smooth, uniform, scratch-free, and mark-free, coats more diificult.

In addition to the web itself, the rod element which removes the excess of a coating composition from a traveling web is customarily resiliently mounted and subject to factors which create variable operation of the composition removal and smoothing device. Thus, for example, coating composition tends to harden or set and to adhere to the rotating element and to hinder element action as well as to cause passage of hardened particles in the element action. As dried, or partially dried, coating composition material builds on the mechanism, the efliciency of the element itself as well as the probability of hardened particle passage increases.

I have found that by appropriately positioning and supporting a small diameter reversely rotating rod element with respect to other components of a coating device, the foregoing difiiculties may in large measure be overcome, that substantial absorption of moisture by a traveling paper web may be avoided during the period when the web is required to be in a strong state, that hygroexpansivity of the web and web wrinkles are substantially eliminated, and that the rotational element may, in effect, flush itself free of hardened particles as well as preventing the buildup of hardened material on stationm parts of the coating removal mechanism. Thus, the mechanical action on the coated web is exerted before any substantial transfer of water to the Web and while the web itself is resistant to stretching and tearing; the resultant coating is then more free of surface defects also.

In the preferred practice of the invention, a resiliently covered backing roll and a coating applicator roll form a coating application nip through which the web, traveling on the backing roll, is directed to receive an excess of coating composition. Such coating application nip may be a positive pressure nip as is usual when coating roll trains are employed or the nip may be an open gap. In eitherevent the coating applied in excess to the web is carried at web speed on the backing roll and directed into contact with a small diameter reversely rotating rod pressured into contact with the web on the backing roll. This rod defines with the backing roll a second nip and the spacing of the two nips around the periphery of the "Ice backing roll is an important consideration in the practice of the invention.

Also, the mechanism which retains this rod, in the practice of the invention, is preferably so disposed that a relatively large obtuse angle is formed between a tangent to the backing roll at line of rod contact and the mechanism. Thus, the coating impelled against the rod, and, more specifically, the excess removed by the rod action, is substantially unconfined and has a large degree of freedom of movement to permit it to flow from the web and the rod support mechanism. The rod itself is also suitably positioned closely adjacent the zone of application of coating composition; the composition is then substantially unset and highly fluid, and the impact with the rod rotating in the reverse direction to coating movement provides for sharp directional change in the flow of the excess coating composition, directing it away from the web and exposed surfaces and particularly away from the rod support mechanism in a sheet or curtain flow. Additionally, the angular arrangement of the rod retaining mechanism is such that the impact of the coating is absorbed without material positional variation in the rod of the device.

In practice, the tendency of absorbent webs to take up water varies with the specific nature of the web; a sized web for example has less tendency to absorb water quickly than does an unsized web. Also, the specific nature of the aqueous coating composition influences the rate at which water is given up by the composition to the web; for example, compositions employing a latex binder such as an acrylic tend to pass water to the web somewhat more quickly than do compositions containing starch or protein binders. I have found that, by appropriately positioning the cylindrical reversely rotating rod relative to the coating application nip so that the rod fronts on the exit side of the application nip, paper webs with coating composition thereon may be directed to the rod without material absorption of water by. the web; thus, the composition as it engages the rod has substantially the same constitution as does the composition applied at the applicator nip. Under some conditions where the tendency of the Web to take up water is high and the tendency of the coating to yield its water content is also high, as in the case of acrylic binder compositions on relatively porous webs, the rod may be positioned substantially in the application nip. The peripheral distance around the backing roll between nips is then very smallsubstantially only one or two degreesof are on the backing roll. With usual paper webs, unsized, the length of are between nips for conventional protein or starch binder-clay coatings may be considerably greater-up to 30 of backing roll are or about one-twelfth of roll circumference. The precise optimum spacing between nips is, of course, affected to some degree by web speed; such speed is usually relatively high in present day coating operations, Whether on or off the papermaking machine, and the effect of speed in a given operating installation may be readily taken into account to secure optimum nip spacing.

The doctor r-od itself is required to be of small diameter to achieve good coating lay on the coated web; the rod diameter is in fact such that the peripheral spacing between nips in the direction of web travel is equal to up to 20 or 30 times rod diameter.

Additionally, the rod support mechanism preferably extends horizontally and not more than about 30 to each side of the horizontal; this provides, in conjunction with the rod position on the backing roll, for the exertion by the coating engaging the rod of a component of fluid force which urges the rod toward the backing roll.

It is, accordingly, a primary object of the present invention to provide a novel coating device which is of particular utility in the coating of absorbent material, such as paper webs.

The invention will be more fully understood by reference to the following detailed description and accompanying drawings wherein:

FIG. 1 is a schematic view in end elevation illustrating one embodiment of the invention;

FIG. 2 is an enlarged and fragmentary view including the principal components of FIG. 1 but having an open coating application nip;

FIG. 3 is a view similar to that of FIG. 1 but illustrating another embodiment of the invention;

FIGS. 4- and 5 illustrate still further embodiments of the invention;

FIG. 6 is a view incorporating features of the invention but with the reversely rotating rod well displaced from the coating application nip; and

FIG. 7 is a schematic and fragmentary view, enlarged, of the relationship of the principal components, and particularly illustrating coating flow.

In the following detailed description and accompanying drawings corresponding numerals, where convenient, designate similar parts.

Referring to the drawings more in detail, the numeral 1 in FIG. 1 generally indicates a train of coating rolls designated at 2 and 4. A pond 6 of coating composition is retained between rolls 2, 4 and coating composition is supplied from an inlet 7 to the pond 6. Rolls 2 and 4 are usually driven at approximately identical rotational speeds in the direction indicated by the arrows; however, the rolls are individually controlled and can be varied in rotational speed approximately 25% either above or below the speed of an applicator roll indicated at 8; in their rotation the rolls 2, 4 form a smooth surface film, in a manner known to the art, and pass the film of coating composition to the applicator roll 8. A trough 9 of any convenient form is arranged to receive coating composi- .tion which is returned to the coating application system in the operation of the coating device. This excess coating may be returned to the coating system at any desired system location.

The applicator roll 8 forms a coating application pressure nip 10 with an upper resiliently surfaced backing roll 14. The lower applicator roll, which customarily is also resiliently surfaced, rotates in the direction indicated by the arrow and carries the excess of coating composition in film form from the metering rolls 2, 4 for delivery to the nip 10. Some small quantity of composition may be retained by applicator roll 8 after passage of the nip, and a doctor blade 12 may bear against the roll on the outlet side of the nip to remove such retained coating from the applicator roll.

The backing roll 14 and applicator roll 8 rotate in the directions indicated by the arrows, that is, in opposite rotational directions. The backing roll 14 carries a paper web 15 to be coated to the nip 10. Preferably the web 15 is applied to the backing roll 14 .at a line of contact 16 well in advance of the nip 1%; this avoids web flutter in the nip and provides for more uniform application of the composition at the nip. However, with appropriate tension on the web, the web may be passed directly into the nip in a horizontal plane if so desired.

The web 15 (FIG. 1) is passed to the backing roll over a guide roll 17; the web is suitably tensioned, as it passes through the paper machine to the backing roll, by conventional devices, the web being drawn to avoid wrinkling, slackness, etc. The web is similarly suitably passed to the backing roll of an oif-the-machine coater where such is employed.

The coating composition carried on the applicator roll is applied to the web in nip 10 in considerable excess of that required in the finished web. However, this excess is impressed on to the web at the nip and at this stage is fluid. Thus, as the web clears nip 10 on the outgoing side, the coating is in the form of a generally smooth fluid film.

Fronting on nip 10, on the outgoing side, is a small diameter, cylindrical, doctor rod 19 of chrome plate steel and which is supported for rotation in a socket 20. Cylindrical rod 19 extends lengthwise of the resilient backing roll 14-, and is retained in socket 20 by spring pressure exerted by spring clip 21 urging the rod into the seat 22. The rod 1 of itself has a degree of flexibility lengthwise which permits it to conform to the backing roll under the influence of the spring pressure of a spring mount 23. The combination of seat and clip form the socket which is itself carried by the spring mount 23 in the form of a longitudinally extending plate. The socket extends only partially around the rod circumference, and the rod area 24 between the clip and seat engages the backing roll 14, forming a pressure nip with the backing roll. The rod rotates counter or reversely to the backing roll as indicated by the arrow. A similar socket is described in Patent No. 2,729,192; another rod retaining means of magnetic nature is described in Patent No. 2,774,329.

The socket 20 is retained on a forward edge of spring mount 23 which extends longitudinally with the socket and which rearwardly is itself supported by a horizontally extending rigid holder 26. A backing plate 27 spot welded to the mount extends lengthwise and rearwardly with the mount and serves to reduce the spring mount resiliency. If desired, the mount 23 may be relatively rigid.

The holder 26 is itself of conventional construction, having an upper jaw 28 and a lower jaw 29 (FIG. 2) between which the resilient mount is clamped. Spaced endwise of the holder (FIGS. 1 and 2) is a suitable rigid bearing support 30 for receipt freely therethrough of an integral pivot shaft 32 of the holder. One such pivot is, of course, provided at each holder end so that the holder assembly, including the rotating rod and its spring mount, may be rotated into pressured engagement with the backing roll. To effect such pivotal movement, the holder is apertured at least at one end rearwardly as at 34 to receive freely therethrough a piston rod 36 of a fluid cylinder 38. Spaced nuts 40, 42 retain the rod with respect to the holder to provide for withdrawal and application of pressure as desired. Accordingly, actuation of the piston rod in one direction pivots the holder to raise the rotating rod 19 and to pressure the rod against the resilient backing roll 14 (FIG. 1). Piston rod actuation in the opposite direction withdraws the rotatable rod from the backing roll.

For effecting total removal of the holder device and components from the area of the backing roll, the cylinder 38 may be disconnected from the holder by releasing nuts 40, 42 and by removing the bearing supports 30 to release the holder. Such specific mechanical arrangement forms no part of the present invention and provision for assembly and disassembly may be effected in any convenient manner. However, it is important that, in operating position, the holder be rigidly supported against movement away from the backing roll.

The rod 19 is itself rotatably driven independently of the backing roll and in the reverse rotational direction to the backing roll 14, as indicated by the arrow in FIG. 2. The rotational speed of this rod is desirably much lower than that of the backing roll.

The rod 19, the mount 23, the pivot arrangement at 32, and the holder 26 lie (FIG. 1) substantially horizontally and at very nearly right angles to a line between the centers of rolls 8, 14 and the nip 10. Thus, the rod is inhibited against movement horizontally away from the backing roll by the rigidity of the structure. Positioning the rod-holding mechanism in such manner that the holder projects rearwardly from the rod and away from the application nip outlet side, and so that the hold er forms an angle which is not greater than about 30 with the horizontal, is eflective to achieve desired operating conditions.

The force urging the rod to the backing roll (FIG. 1) is applied substantially parallel to the line between roll centers. Thus, a minimum only of force is required to secure a definite, determinable pressure between the rod 19 and the backing roll. The minimum force necessary would be achieved when the force is applied substantially radially toward the center of backing roll 14.

In operation, the backing roll 14 and the web move at a fairly high speed of 500 to 2000 f.p.m.; 600 f.p.m. is exemplary at a diameter of 30 inches for the backing roll. The applicator roll described conventionally is driven at a speed of about 590 f.p.m. or slightly slower than the backing roll when nip 10 is a pressure nip (FIG. 1). The rod 19 has a suitable speed range of -150 r.p.m., a rotational speed of 100 rpm. GA" diameter) being suitable when the backing roll is at 600 f.p.m. as above.

Under these conditions, with nip 10 having a lineal pressure of about 3 to 15 pounds per lineal inch, the composition 6 is applied in excess to the moving web 15 but the excess itself is smoothed on the web. The coated web passes from the nip 10 to the rod nip in about 0.016 second (600 f.p.m. at 2" spacing). This dwell time is sufliciently short that the excess of coating retains its film and smooth form as it approaches the rod, and substantially no absorption of water by the web occurs. The peripheral distance of the second and pressure nip around the backing roll from the application nip in such instance is about eight times rod diameter. The rod serves to split the film of coating compositionone portion being carried with the web on the backing roll 14 and being subjected to shearing action by the rod, and the other portion sheeting abruptly downwardly under the influence of rod rotation and gravity (FIGS. 1 and 2) in a curtain 11 of coating composition.

There is substantially no splashing of coating and no opportunity for hard particles to lodge between the rod and backing roll or to adhere to the socket or other components of the equipment. Also, since the composition which sheets ofi has substantially its original fluidity, unchanged from the application nip condition for practical purposes, the flow is constant and relatively smooth. More importantly, the fluid pressure exerted against the rod is in very nearly the same plane in which the rod is supported and substantially perpendicular to the plane in which the rod is pressured. The fluid pressure effect is thus absorbed by the combination of the rod support mechanism and the backing roll itself; in addition, since the coating composition is still highly fluid and has sub-. stantially the fluidity of the original composition, it pools slightly in front of the rod, changes direction readily, and flows downwardly under the influence of the rotating rod and gravity, oil" the front of seat 22, and then in a sheet or curtain to the applicator roll (FIG. 1). Importantly also, the force application by the composition is such that wear of the rod in the socket is minimized.

Additionally, the coating composition retained on the Web, even at high solids and viscosity of the coating, is sufiiciently fluid after passing the rod that surface tension effects tend to cause the coating to flow together on the web, thereby eliminating tracks and other marks which may sometimes occur as the rod 19 wears.

The arrangement of FIG. 2 is similar to that of FIG. 1 except that the rubber covered backing roll 14 and the applicator roll 8 form an open nip or gap 46. With such an open gap, the rod may be positioned very closely to the applicator nip, the close positioning being limited only by the necessity for suflicient clearance for passage of the excess coating material from the coating pool acted on by the rotating rod. The applicator roll 8 (FIG. 2)

6 in this instance rotates in a pond of coating similar to that illustrated in FIG. 3 and preferably at a lower speed relative to the backing roll as described in connection with FIG. 3.

In FIG. 3 the applicator roll 48 rotates in a trough 50,

and carries an excess of coating to the web 15 on the backing roll 14 at the open nip 53. The gap of open nip 53 may have a dimension as great as 0.060". In general, however, it is desirable to maintain the gap not greater than about 0.020" to 0.040".

In the arrangement of FIG. 3 the applicator roll and backing roll are each independently driven as described in FIG. 1, but, in this instance, the applicator roll may rotate at a much lower speed, that is, approximately f.p.m. when the backing roll is at 600 f.p.m. In such instance the backing roll may have a diameter of 30" while the applicator roll may have a diameter of about 12". The applicator roll forms the coating application nip with the backing roll at such a distance in advance of the pressure nip that the curtain designated at 56 drops on to the applicator roll and then into the pond 54.

The arrangement of the holder and rod in the structure of FIG. 3 is similar to that of FIG. 1 but the rod is biased to the backing roll by a somewhat diflerent mechanism, the holder itself projecting toward the application nip generally horizontally in operating position.

In FIG. 3 the rod 19 is carried on an edge of the spring mount 23 which is itself retained by holder 57 which itself has an upper portion 58 secured by a bolt 59 to a lower portion 60, the mount 23 being retained between the portions. The holder is pivoted at 61 on a pivot shaft positioned beyond the end of trough 50, and which pivot shaft lies between the applicator roll and a line passed through the center of the backing roll 14 and the center of the rod 19. Under this condition actuation of piston rod 62 of cylinder 63 serves to move the holder 5'7 in a positive action relative to the backing roll.

In FIG. 4 the backing roll 14 and the applicator roll 48 are offset so that the center line passing through the centers of the backing roll and applicator roll is at an angle with the vertical and also with the horizontal. In this instance the rod 19 is positioned substantially vertically below the center of the roll 14, that is, 0 degree around the periphery from the vertical diameter in contrast to the arrangement of FIG. 1 where-it may be up to 30 of backing roll are or around the periphery from the vertical diameter and coating application nip. The preferred structural arrangement of the holder of FIG. 4 is similar to that of FIG. 1 and corresponding numerals apply to the holder device. In FIG. 5 the structural arrangement is similar to that of FIG. 3 except that the holder 26 is inclined at an angle a to the horizontal. The angle which the holder can make with the horizontal and still obtain the curtain of coating from the seat of the rod holder 22 depends somewhat on the particular coating composition being used. The angle a for the exemplary coating, set out hereinafter, which would be allowable would be approximately 30 to the horizontal. The curtain of coating composition 56 flows directly downwardly and the plate 23 and the holder itself are not fouled by the coating composition.

In FIG. 6 the components bear similar legends to the arrangement of FIG. 5 but the holder is disposed at an angle above the horizontal and suitably forms with a tangent to the backing roll at the line of contact with the rotating rod 19 an angle of up to about This arrangement is useful with coating compositions which do not readily wet the web.

The coating composition which is customarily employed With the foregoing apparatus arrangements in the. coating of paper webs is customarily an aqueous mineral coating composition. The binder of such coating composi tions may, however, be a conventional binder such as starch, casein, or the like, or may be a latex such as the acrylic latices. An exemplary coating composition useful in any of the foregoing described embodiments is as follows:

It is to be noted that the coating composition carried on the web in excess impacts against the rotating rod rather strongly, developing considerable pressure on the rod which is resisted by the structural arrangement described. The rotation of the rod itself in the reverse direction aids rapid halting of the excess portion of the composition. Further, the fact that the dwell time is short and the composition is quite fluid aids halting of the composition flow and change of its direction. Additionally, the fluidity of the composition provides for the falling away of the composition in the curtain or sheet formed. This not only tends to Wash the area around the rotating rod clean but, additionally, prevents accumulation of dried coating on the mounting 23.

Too large a rod diameter is undesirable as it leads to coarse coating lay in the finished coating and is not quite as effective as the smaller rods in forming the curtain.

The specific nature of the webs which may be employed influence only slightly the specific position of the rod with relation to the applicator nip. Thus, a web of 25% sulfite short fiber, short fiber kraft, and of long fiber kraft may be formed on the conventional Fourdrinier papermaking machine, dried, and smoothed into a pound uncoated sheet and passed in a continuous operation to the coating device. Coat weights of 8-10 pounds per side employing the specific composition set out hereinbefore may be achieved. Such web and coating is, however, not critical and may be used with any of the embodiments described.

The pressure exerted by the rod against the backing roll is likewise not critical, pressures in the range of 215 pounds per lineal inch being useful. Higher pressures tend to produce lower coating composition weights on the sheet, other conditions being equal. The rod action tends to smoothly remove the outermost portion of the composition from the web; since there is little time for migration of water from such zone of the composition, the coating sheeted off in the form of the curtain is the same in composition as that applied at the application nip. The coating composition may therefore be and is returned to the coating system unchanged, and is again applied to the traveling web.

The portion of the composition which passes in the rod pressure nip in the form of a fluid film is pressured into the web, which then passes to the dryers. Dryer action is also aided since the web has taken up a lesser quantity of water than in conventional procedures.

It will be understood that this invention is susceptible to modification in order to adapt to different usages and conditions and, accordingly, it is desired to comprehend such modifications within the invention as may fall within the scope of the appended claims.

What is claimed is:

1. In combination, in coating apparatus for paper webs and the like wherein an aqueous hardenable mineral coating composition is applied to a traveling web: an upper resiliently covered rotatable web carrying backing roll and a lower rotatable applicator roll defining with the backing roll a coating application nip through which a web to be coated is passed from a coating application nip inlet to a coating application nip outlet to receive an excess of aqueous coating composition, said application nip lying on a vertically extending center line between the backing and applicator rolls; a small diameter doctor rod rotatable counter to the backing roll independently of the backing roll fronting on the outlet side of said application nip around the backing roll from said nip and forming With the backing roll a second and pressure nip through which the web with coating thereon is passed; the said second and pressure nip being from about 2 to not more than 30 of are around the backing roll from the coating application nip and which latter and greater distance is equal to up to 20 to 30 times the diameter of the said doctor rod, the said doctor rod also lying within from about 0 to 30 around the backing roll periphery from the vertical diameter of the backing roll and in the direction of backing roll rotation, and support mechanism for said doctor rod including a socket supporting said rod and a holder supporting said socket, said holder projecting rearwardly from said socket and rod and away from said application nip outlet side and forming with the horizontal an angle of less than 30.

2. In combination, in coating apparatus for paper webs and the like wherein an aqueous hardenable mineral coating composition is applied to a traveling web: an upper resiliently covered rotatable web carrying backing roll and a rotatable applicator roll positioned directly below the backing roll forming with the backing roll a coating application nip through which a web to be coated is passed from a coating application nip inlet to a coating application nip outlet to receive an excess of an aqueous coating composition, said application nip lying on the vertical center-line between the backing and applicator rolls; a small diameter doctor rod rotatable counter to the backing roll independently of the backing roll fronting on the outlet side of said application nip around the backing roll from the application nip and forming with the backing roll a second and pressure nip through which the web with coating thereon is passed; the said second and pressure nip being from about 2 to not more than 30 of are around the backing roll from the coating application nip and which latter distance is equal to up to 20 to 30 times the diameter of the said rod, the said rod also lying within from about 0 to 30 around the backing roll periphery from the vertical diameter of the backing roll and in the direction of backing roll rotation, and support mechanism for said doctor rod including a socket supporting said rod and a holder supporting said socket, said holder projecting rearwardly from said rod and away from said application nip substantially horizontally in a direction perpendicular to the said vertical centerline of the backing and applicator rolls.